The present invention relates to a cooling cycle which allows a gas to be liquefied as a result of the regasification of liquefied natural gas. The invention also relates to a method of fractionating air by liquefaction and distillation, which employs a cooling cycle according to the invention to liquefy at least one gas fraction, such as gaseous nitrogen, resulting from the fractionating process.
The technical and economic context of the present invention is as follows:
Now that certain countries (in particular, the United States, Western Europe and Japan) have become vast consumers of energy, increasingly large amounts of natural gas have to be transported from producing countries (in particular the USSR, Africa, South America, etc.) to such consumer countries.
When the distances over which the natural gas has to be transported are excessively great, and when sea transport is feasible or inevitable, as it sometimes is, (for intercontinental transport), the natural gas is liquefied in the producing country, the liquefied natural gas (hereinafter referred to as LNG) is transported in ships designed for the purpose (methane carriers), and the LNG so transported is converted back into gas in the consumer country.
To this end, the following operations are carried out in the consumer country:
1. The LNG transported is received at specially designed harbour installations generally referred to as "terminals". PA0 2. The LNG is stored in thermally insulated tanks at the terminals. PA0 3. The LNG is regasified and compressed in suitable units at the terminal; the regasification may be accompanied by treatment of the LNG for the purposes of reducing its calorific value and/or extracting certain usable fractions which can be consumed separately (such as propane, and butane). PA0 4. The regasified LNG, or natural gas (hereinafter referred to as NG), is piped from the terminal to places where it is used and consumed. It is generally piped at a high pressure (30 to 80 bars). PA0 1. compressing a refrigerant (nitrogen for example) in gaseous form from a low pressure to a high pressure in one or more compression stages, PA0 2. in cases where the high pressure is higher than the critical pressure of the refrigerant, cooling the compressed refrigerant, or, in cases where the high pressure is lower than the said critical pressure, cooling and then condensing the compressed refrigerant by means of heat exchange with the above-mentioned cold source (liquefied natural gas in course of regasification, PA0 3. de-pressurising the cooled or condensed refrigerant isenthalpically in one or more de-pressurising stages, in other words bringing about a Joule-Thompson type de-pressurisation of the cooled or condensed refrigerant in one or more stages, in order to obtain a condensed refrigerant at low pressure, PA0 4. when the cycle is a closed one, vaporising the condensed refrigerant and then heating it by heat exchange with the above mentioned cooling load (a gaseous fraction or gas to be liquefied in course of cooling and then liquefaction). PA0 1. if consideration is restricted on the one hand to LNG in the course of vaporisation at a pressure lower than its critical pressure, and on the other hand to nitrogen in the course of condensation at a pressure lower than its critical pressure, the enthalpy curve (enthalpy H on the y- axis and temperature T on the x- axis) for LNG in the course of vaporisation is in the form of a long thermal gradient, while the enthalpy curve for nitrogen in the course of condensation is of a vertical nature. Under these circumstances it is in general difficult to achieve a match between the enthalpy curve for the refrigerant and on the one hand the special enthalpy curve for LNG and on the other hand the enghalpy curve for nitrogen, (i.e., ensure the minimum of irreversibility). PA0 2. the Joule-Thomson de-pressurisation or de-pressurisations mentioned above are very far from being reversible. They in fact involve an expansion or change of volume on the part of the nitrogen which is de-pressurised, either because the nitrogen is de-pressurised in the gaseous or super-critical form or because it is de-pressurised in the liquid form and cannot be sufficiently sub-cooled before de-pressurisation thus causing a by no means negligible "flash" to be produced in the course of de-pressurisation. PA0 1. the cooling known as the "incorporated cascade", or "single-stream cascade", or "mixed refrigerant cascade", or again "mixed refrigerant", cycle may be used to cool liquefy, and possibly subcool natural gas (NG). PA0 A. "gas to be liquefied": as examples, the following fall within the definition: a gaseous mixture consisting of a plurality of pure components or substances, a pure gas consisting of only one pure component or substance which it is desired to condense wholly or partially, and substantially pure gaseous nitrogen or a substantially pure gaseous nitrogen fraction produced by distilling air. In cases where the gas to be liquefied is a mixture of gases, the mixture may be condensed in a fractional manner. PA0 B. "cycle mixture": a mixture consisting of a plurality of pure components or substances which may or may not be physically identifiable, flowing in circuit in an incorporated cascade cooling cycle, the sole function of which is cyclically to extract cooling energy from the cold source (liquefied natural gas in course of regasification) and to transfer the refrigeration extracted from the cold source to the cooling load (gas to be liquefied in the course of cooling), PA0 C. "external refrigerant": when not liquefied natural gas, a refrigerant from a source external to the incorporated cascade cooling cycle. Inter alia, such an external refrigerant may be used firstly to pre-cool the gas to be liquefied, secondly to cool the cycle mixture when in the gaseous state and compressed at high pressure and, before its fractional condensation, and thirdly to cool the cycle mixture when compressed at the high pressure with a view to assisting its partial, preliminary condensation (the beginning of the fractional condensation) which takes place in accordance with the inventiion by heat exchange with LNG in the course of regasification. Such an external refrigerant may be a liquid such as water in the course of being heated, or a liquid refrigerant such as propane in the course of vaporisation at one or more vaporisation pressures. In the latter case any other external refrigerant equivalent to propane may be used. This may for example mean a mixture of pure substances (such as propane and propylene) or one and the same pure substance (such as butane). It may also mean ammonia or the fluorinated hydrocarbon refrigerants known as "Freons". Also in the latter case, the incorporated cascade cooling cycle may co-operate with another cooling cycle, or auxiliary cooling cycle, which allows the external refrigerant to be recondensed after evaporation and which involves successively compressing the external refrigerant after vaporisation, condensing the compressed external refrigerant by heat exchange with another external refrigerant such as water, de-pressurising the condensed external refrigerant, and vaporising the de-pressurised external refrigerant by heat exchange for example with the compressed cycle mixture at the high pressure before its fractional condensation, the said evaporated external refrigerant being then cycled back to compression, PA0 "COMPOSITION": unless indicated to the contrary, the composition in terms of volume of a gas or liquid which is imagined to be totally vaporised, expressed in molar or volumetric percentages, PA0 "COMPOSITION OF THE CYCLE MIXTURE": the composition in terms of volume of the cycle mixture. In the case of an incorporated cascade cycle of the closed type the composition which is analysed and measured may be that of the compressed cycle mixture in the high pressure gaseous state, before its partial preliminary condensation (the beginning of the fractional condensation by heat exchange with LNG in the course of regasification. In the case of an incorporated cascade cycle of the open type, the cycle mixture proper cannot be measured and analysed as such. In this case the composition of the cycle mixture may be calculated by adding up the quantities of the various components of the cycle mixture contained in the various condensed fractions of the said mixture which are de-pressurised to the low pressure of the cycle and returned to the point where the cycle mixture is compressed, PA0 "TO COOL" or "COOLING": unless indicated to the contrary in any particular case any operation performed on a gas by which heat is extracted from the said gas. Such cooling involves at least one of the following processes when the said gas is at a pressure lower than its critical pressure: PA0 G. "fractional condensation": an operation consisting of at least: PA0 "TO HEAT" and "HEATING": unless otherwise indicated in any particular case, any operation carried out on a liquid or a two-phase mixture of liquid and gas, by means of which heat is given up to the said liquid or to the said two-phase mixture. Such heating involves at least one of the following processes: PA0 I. "regasify" and "REGASIFICATION": the act of adding heat to liquefied natural gas with the object of converting it from the liquid state to the gaseous state. When the liquefied natural gas is regasified at a pressure lower than its critical pressure, the regasification consists of heating as defined above. PA0 J. "refrigerant stream" and "COOLING STREAM", streams of the cycle mixture which are intended to cool respectively the said cycle mixture in the course of fractional condensation, and the gas to be liquefied. These streams both flow from the cold end to the hot end of a heat-exchange assembly and initially, that is to say at the cold end of the said heat-exchange assembly, are the result of introducing at least a de-pressurised part of a condensed fraction of the cycle mixture into the said heat-exchange assembly and then vaporising it, which fraction, in the course of the progress of the said stream towards the hot end of the said heat-exchange assembly, is joined by at least a de-pressurised part of at least one other condensed fraction of the cycle mixture.
Generally, to regasify the LNG it is compressed in liquid form from its storage pressure (atmospheric pressure) to the supply pressure of the network for distributing the natural gas and it is then vaporised at this supply pressure. The heat required to vaporise the gas and then heat it to ambient temperature is provided by sea water. Compressing the LNG by means of pumps in the above way allows a double economy, both in capital investment and in the energy consumed by the regasification plant.
However, rather than lose for good the potential cooling energy contained in LNG, namely the amount of cooling energy which can be extracted from it when it is regasified, particularly by vaporising it and then heating its vapour, it is preferable to use at least a part of this cooling energy for useful purposes.
It has therefore been proposed that the lowest temperature, i.e., the "highest grade", cooling energy from the LNG should be used in installations for fractionating atmospheric air by liquefaction and distillation, with a view to obtaining at least one of the gaseous fractions produced, namely oxygen or nitrogen, in the liquid state. In effect, using the cold or cooling energy from the LNG allows the energy expended on liquefying such gaseous fraction or fractions to be reduced.
In general terms, to transfer the potential cooling energy contained in the LNG to the gaseous air fraction or fractions to be liquefied, a cooling cycle of the open or closed type is used in which the refrigerant may be nitrogen for example. Like all cooling cycles, this cycle may be looked upon as allowing:
Cooling energy to be extracted from a cold source (LNG in course of regasification) at graduated temperatures between an initial temperature (the temperature at which refrigeration begins to be taken from the LNG), which is at least equal to -161.degree. C, and a final temperature (the temperature at which refrigeration ceases to be extracted from the LNG), which is at most equal to ambient temperature, for example 0.degree. C.
The refrigeration extracted from the cold source to be transferred to a cooling load (a gaseous fraction, or gas to be liquefied, which is in the course of being cooled, then liquefied, and possibly sub-cooled) at graduated temperatures between an initial temperature (the temperature at which the gaseous fraction or the gas to be liquefied begins to be cooled), which may be ambient temperature, and a final temperature (the temperature prevailing at the conclusion of the liquefaction and possibly the sub-cooling of the gaseous fraction), which is generally lower than the initial temperature at which refrigeration begins to be extracted from the LNG.
In general and simplified terms, such a cooling cycle consists of:
In the case of an open cycle (such as the liquefaction of gaseous nitrogen by means of a refrigerant formed by nitrogen), stage (4) defined above can be dispensed with since condensed gaseous nitrogen can be tapped off from the condensed refrigerant obtained in the course of stage (3) above.
From a purely thermodynamic point of view, the cooling cycle described above is irreversible at a substantial number of points, as is explained below in the case of the liquefaction of nitrogen by means of a refrigerant formed by nitrogen:
For these two reasons, the effectiveness of the cooling cycle described above cannot be considered optimum.
The present invention thus has as a main object a cooling cycle which is different from that described above and which is particularly suited to the transferring cooling energy from a cold source formed by LNG in course of regasification to a cooling load formed by a gas to be liquefied which is in the course of being cooled, then condensed, and possibly sub-cooled, which gas to be liquefied may be a gaseous fraction resulting from fractionating air by liquefaction and distillation.
To be more exact, the present invention has as a main object a cooling cycle which enables the two thermodynamic irreversibilities mentioned above to be overcome.
The present invention is based on the following considerations and facts:
Such a cycle was for example the subject of a French Patent No. 1,302,989 filed in the name of L'Air Liquide and of its first certificate of addition no. 80,294 and its second certificate of addition no. 86,485.
Such a cycle may be defined in general terms as consisting of at least the following basic operations:
a. At least one cycle mixture, in gaseous form, consisting of a plurality of components is compressed from a low pressure to a high pressure, the compression taking place in at least one compression stage, PA1 b. At least the compressed cycle mixture is subjected to fractional condensation at the said high pressure, fractional condensation comprising at least: PA1 refrigeration to be extracted from a cold source formed by the said external refrigerant, which consists of a single component and is in the course of heating (water for example), or in the course of vaporisation at one or more vaporisation pressures, (propane for example), and thus refrigeration to be extracted at one temperature and one only (as in the case of propane in the course of vaporisation at a single vaporisation pressure for example) or at temperatures distributed along a relatively short temperature gradient (as in the case of water in the course of heating of propane in the course of vaporisation at a plurality of vaporisation pressures for example), PA1 the refrigeration extracted from the hot source to be transferred to a cooling load (NG in the course of being cooled, and then liquefied and possibly sub-cooled) at temperatures distributed along a relatively long temperature gradient (in order to cool, and then liquefy and possibly sub-cool the NG), PA1 the cold source for the cycle is now formed by LNG in the course of regasification and not by an external refrigerant such as water or propane. Consequently, the refrigeration is now extracted from cold source at temperatures which are distributed along a relatively long temperature gradient from at least the temperature at which the LNG begins to be regasified to the temperature at which this regasification ends, PA1 the cooling load for the cycle is now formed by the gas to be liquefied when in the course of cooling and then condensation and possible sub-cooling, and not by NG to be liquefied. Consequently, in cases where only a pure gas, i.e., one consisting of only one component, is condensed, the refrigeration transferred to the cooling load is extracted at one single temperature, or along a relatively short temperature gradient. PA1 as explained above, the enthalpy curve for the fractional condensation of the cycle mixture can be matched to the enthalpy curve for the regasification of the LNG, PA1 as in any incorporated cascade cycle, the Joule-Thomson de-pressurisations may be performed on sub-cooled liquids and thus with virtually no expansion of the fluid which is de-pressurised. PA1 1. cooling the said gas from an initial temperature near to or lower than ambient temperature to a final temperature equal to or higher than the dew point of the said gas, the gas remaining in the gaseous state, PA1 1. wholly or partly condensing the said gas when initially it is at its dew point. In cases where a gas consisting of only one component is wholly or partly condensed, condensation takes place at a substantially constant temperature. In cases where a gas consisting of a plurality of components is wholly or partly condensed, condensation takes place by lowering the temperature of the said gas from its dew point to a temperature higher than or equal to its boiling point. In cases where a gas consisting of a plurality of components is condensed, condensation may take place in a fractional fashion, PA1 3. sub-cooling the condensed gas, or at least a condensed fraction of the said gas in cases where the gas has been subjected to fractional condensation, the sub-cooling taking place by lowering the temperature of the said gas, or the temperature of at least the said condensed fraction, from an initial temperature close to the boiling point of the said gas or the said condensed fraction to a final temperature. PA1 a. a first fractional condensation stage during which a gas (such as the cycle mixture in the compressed gaseous state) is partially condensed by heat exchange with at least one refrigerant. The partially condensed gas is separated into a first condensed fraction, and a first vapour fraction which continues with the fractional condensation, PA1 b. possibly at least one intermediate fractional condensation stage during which the first vapour fraction, or a vapour fraction from the preceding stage of fractional condensation, is partially condensed by heat exchange with at least one refrigerant. The vapour fraction which is partially condensed in this way is separated into a second condensed fraction, or penultimate condensed fraction, and a second vapour fraction, or last vapour fraction, which continues with the fractional condensation, PA1 c. a last fractional condensation stage during which the last vapour fraction is wholly condensed by heat exchange with a refrigerant, as a result of which a last condensed fraction is obtained. PA1 1. entire or partial vaporisation of the said liquid or the said two-phase mixture, which is initially at the boiling point of the said liquid or the said two-phase mixture. When the said liquid contains a plurality of components, or when the said two-phase mixture contains a plurality of components, during this entire or partial vaporisation the temperature of the said liquid or the said two-phase mixture is increased from the said boiling point to a temperature lower than or equal to the dew point of the said liquid or the said two-phase mixture. When the said liquid or the said two-phase mixture is formed by a single pure substance vaporisation takes place at one and the same temperature. PA1 2. heating of the said liquid once vaporised or the said two-phase mixture once vaporised from a dew point of the said liquid or two-phase mixture once vaporised to a final temperature close to or lower than ambient temperature. PA1 In the case of an incorporated cascade cycle of the open type, the bringing together of at least a part of the gas to be liquefied and at least a part of the cycle mixture may take place either at the low pressure, for example at the point where the cycle mixture is drawn in for compression, or at the high pressure, for example at the point where the cycle mixture is delivered from compression or in the course of fractional condensation of the cycle mixture, or finally at a pressure intermediate between the high and low pressures of the cooling cycle, for example in the course of the compression of the cycle mixture.
b.1 A first fractional condensation stage during which at least the compressed cycle mixture is partially condensed by heat exchange with at least one external refrigerant. At least the partially condensed cycle mixture is separated into a first condensed fraction and a first vapour fraction which continues with the fractional condensation, PA2 b.2 A last fractional condensation stage during which a last vapour fraction of at least the cycle mixture is totally condensed by counter-current heat exchange with a refrigerant stream of the cycle mixture, which is in the course of heating at a vaporisation pressure at least equal to the low pressure. In this way is obtained a last condensed fraction, PA2 c. Parts at least of the first condensed fraction and of the last condensed fraction are de-pressurised from the said high pressure to the said vaporisation pressure, the de-pressurised part of the last condensed fraction forming at least an initial part of the said refrigerant stream, and at least the de-pressurised part of the first condensed fraction being combined with said refrigerant stream, PA2 d. The parts de-pressurised in stage (c) are vaporised and the said refrigerant stream is heated at the said vaporisation pressure by counter-current heat exchange with at least the cycle mixture which is in course of fractionated condensation at the high pressure, PA2 e. The gas to be liquefied is cooled by counter-current heat exchange with a cooling steam of the cycle mixture which is in the course of heating at a heating pressure equal to the said low pressure, and at least part of the said gas to be liquefied is withdrawn in the condensed state as liquid product, PA2 f. At least the refrigerant stream which has been heated in stage (d) is recompressed as in stage PA2 a. from the said vaporisation pressure to the said high pressure to again form at least part of the cycle mixture at the high pressure. PA2 In the case of the fractional condensation of the cycle mixture, the number of fractional condensation stages is equal to the number of separating drums which separate a condensed fraction and a vapour fraction, plus one.
In a similar way to the cycle discussed above, this cycle may be looked upon as allowing:
2. from the thermodynamic point of view, the advantage of the incorporated cascade cycle when applied to the liquefaction of NG lies in the fact that, since there are a plurality of components in the cycle mixture, on the one hand the special enthalpy curve for the NG or cooling load (in the course of cooling then liquefying and possibly sub-cooling), whether it is combined (in the case of a closed cycle involving only one vaporisation pressure for the cycle mixture) or not (in the case of a closed cycle involving two vaporisation pressures for the cycle mixture) with the enthalpy curve for the cycle mixture in the course of fractional condensation, and on the other hand the enthalpy curve for the cycle mixture in the course of vaporisation and heating at the low pressure of the cooling cycle, can be made to match (i.e., irreversibility can be brought to a minimum),
3. conversely, if consideration is now given on the one hand to LNG in the course of re-gasification, for which the enthalpy curve is of a similar, if not identical, form to the enthalpy curve for the cooling, liquefaction and possibly sub-cooling of the original NG, and on the other hand to a cycle mixture such as that described above when in the course of fractional condensation at a high pressure, for which the enthalpy curve is similar in form to that for the vaporisation and heating of the same mixture at a low vaporisation pressure, it will be realised that it ought to be possible for a counter-current heat exchange between on the one hand LNG in the course of regasification and possibly a cycle mixture in the course of vaporisation and heating, and on the other hand this same cycle mixture in the course of fractional condensation, to take place with a minimum of thermodynamic irreversibility.